Basically the idea is this:

When the exhaust gas first leaves the heads, it is very hot and expansive. As it moves through the exhaust system it cools and contracts. The pipes should be sized to accomodate this contraction so the gas doesn't lose velocity. This is all well and good up to a point. Namely the first restriction in the system- whether it be a cat or muffler. With the factory system, you have very good flowing exhaust manifolds that go into a tiny and ill bent Y pipe that then dumps into a 3" pipe right before a very restrictive cat. Backpressure indeed. It is set up this way so that the catalytic converter will reach operating temperature (light off) very quickly.

 

thus that's why i said he will lose low end

 

Thus why I said:

 

LOL at bouncer and mopower. I love benchracing. Bench dynos are even funnier. I have to add to this and say its all dependant upon the setup, and you really cant tell unless you actually try it (or you know the formulas and variables to input). In my personal experience my torque was greater ever so slightly when I had open headers, but the top end died horribly. With my exhaust on it pulls well down low, and like an ls1 up top. The 360 motor is quickly becoming my favorite motor for a multitude of reasons.